We have measured the induced surface potential on single crystal of Al2O3 under electron irradiation in scanning electron microscope (SEM), using X-ray spectroscopy method in conjunction with the electron spectroscopy method. A big disagreement between the two methods is observed. The reason of this disagreement is analysed, We show that the position of the experimentally measured high energy cut-off limit of X-ray continuous radiation (bremsstrahlung) detected from the charged sample is not consistent with the effective landing energy of the primary electrons and hence its position is not directly connected with the surface potential of the charged sample.